A call set up between a mobile station and a cellular network when the network is heavily loaded, can fail in many cases due to the fact that the cellular network has a certain upper capacity which restricts the availability of the network. Failed set-ups arise particularly in densely populated regions where call peaks appear at certain times of the day.
The coverage of mobile telephone networks vary geographically. In certain regions, the coverage can be good for one system, while the coverage may be poor for another system.
However, it is in the interest of a cellular network operator to see to it that as many call set-ups as possible succeed. For that reason, many operators of cellular networks desire that it should be possible to use one and the same mobile station for calls in a plurality of cellular networks. These networks can be built up by different types of mobile telephone systems which, for example, can imply that the two systems have different carrier frequencies, which is the frequency that is used upon radio communication between a mobile station and the mobile telephone network. These different types of cellular networks can be supplied by one and the same operator or by different operators. If the possibility exists, always to be able to choose the best network from for example the mobile station, a user can increase his or her availability towards the network, which also implies that more calls can be set up.
Mobile stations usable in several cellular networks built up by means of different mobile telephone systems necessitate in principle a transmitter and a receiver for each of the mobile telephone systems. In order for the mobile station not to become too big, it is required that the same type of components is used for the different systems to such a large extent as possible. This leads to being fewer components being included in the mobile station which means that the mobile station will be cheaper, smaller and lighter.
European patent application EP-678 974 A2 discloses a transmitter and a receiver for radio frequency systems. The transmitter and the receiver are intended to be used for transmission and reception within two different frequency ranges. Common to the transmitter and the receiver is a voltage controlled crystal oscillator which generates a mixer signal LO3 having a frequency equal to 26 MHz. Moreover, two synthesizers are connected to the voltage controlled crystal oscillator. They each generate a mixer signal LO1 and LO2, respectively, by means of the mixer signal LO3 received from the crystal oscillator. The first synthesizer generates the mixer signal LO1 at different frequencies depending on which frequency range is to be transmitted and received. For GSM, the frequency of LO1 is within the 1500 MHz range and for PCN, the frequency of LO1 is within the 1200 MHz range. The synthesizer that generates LO1 comprises two voltage controlled oscillators (VCO) in order for the mixer signal LO1 to be delivered across the interval 1200-1500 MHz with sufficient accuracy. The synthesizer that generates L02 comprises one VCO. Thus, this apparatus comprises three VCOs.
In the transmitter, the I-signal and the Q-signal from the base band unit are I/Q-modulated with a predetermined frequency. This frequency is the mixer signal LO2 divided by a factor N. For GSM, N is equal to one, and for PCN, N is equal to two. The I/Q-modulated signal obtained, is then mixed with LO1, whereby an RF signal is obtained. The frequency content of the RF signal is dependent upon the magnitude of LO1. For GSM, an RF signal within the 900 MHz range is obtained and for PCN, an RF signal within the 1900 MHz range is obtained. Depending upon the frequency range of the RF signal, the RF signal is supplied to different RF parts via a switch. The GSM system has an RF part comprising an RF filter, an RF power amplifier and a duplex filter. The RF part of PCN comprises the same type of components but adapted to the 1900 MHz range.
A disadvantage with this solution is that the apparatus comprises three VCOs in order to be able to create mixer frequency signals that are sufficiently accurate.
Another disadvantage is that the apparatus uses two different RF filters dimensioned for the respective frequency range for filtering the RF signal before transmission.
EP 631 400 A1 discloses a solution for a dual band radio communication device intended for a satellite-based network and a land-based network. The satellite-based system transmits signals within the frequency range 1610-1625.5 MHz and receives signals within the range 2485.5-2500 MHz. The corresponding frequency range for the land-based system is 1710-1785 MHz upon transmission and 1805-1880 MHz upon reception. In a transmitter part in the device, an I/Q-modulator is used for modulating an I-channel and a Q-channel from the base band part of the device. A VCO is connected to the I/Q-modulator to generate an RF signal which is amplified, filtered and then transmitted into the air via an antenna. Due to the fact that the frequency ranges of the RF signal transmitted by the two systems are close to each other in the frequency plane, one and the same VCO can be used in the transmitter part.
A disadvantage of this transmitter part is that the frequency ranges of the transmitted signals have to be close to each other.
A common solution when transmitting in an upper and a lower frequency band is to provide a multiplier having a suitable multiplication factor in the RF part of a transmitter. When RF signals in the upper frequency band are to be transmitted, the multiplier is connected. Then, RF signals are obtained in the upper frequency band in correspondence to RF signals in the lower frequency band multiplied by the multiplication factor.
A disadvantage of this solution is that the modulation bandwidth of the RF signals in the upper frequency band increases relative to the modulation bandwidth of the RF signals in the lower frequency band. This is not acceptable if RF signals are to be transmitted in radio systems having the same channel separation.